An example of the accurate required detection of measurement signals is electronic motor protection, in which the motor currents are used as input variables to a thermal motor model. In order to minimize the number of variants of motor protective devices, motor protection with a wide adjustment range of, for example, 1:10 is required, with this indicating the ratio of the minimum rated current to the maximum rated current of the motor. Furthermore, it is necessary to take account of the fact that the starting current of the motor may be ten times the rated current, so that a dynamic range of 1:100 is required for the analogue/digital converter.
Additionally, current measurement during operation is used to detect whether the motor is switched on or off correctly. On the basis of practical experience, it can be assumed that, when the motor is switched off, the measured current is less than 20% of the motor rated current. This necessitates an increase in the dynamic range to 1:500.
In addition to the analog/digital converter, an appropriate current sensor is also required for detection of the current, and has to cover the same dynamic range. Furthermore, there is a requirement for a low cost current sensor, which can be achieved with a small current sensor with a small output signal. However, this results in the problem that the small output signals such as these may be from the same range as interference signals, such as noise, crosstalk etc, while it is nevertheless necessary to distinguish between the motor current as the actual useful signal and corrupting interference signals.